Piezoelectric ceramic composition

ABSTRACT

The present invention provides a piezoelectric ceramic composition which predominantly comprises a composition represented by the following formula: 
     
         (1-n)(K.sub.1-x-y Na.sub.x Li.sub.y).sub.m (Nb.sub.1-z Ta.sub.z)O.sub.3 
    
      -nM1M2O 3   
     wherein M1 represents a divalent metal element such as Mg, Ca, Sr or Ba; M2 represents a tetravalent metal element such as Ti, Zr, Sn or Hf; and x, y, z, m, and n satisfy the following conditions: 0.9≦X; y≦0.3; x+y&lt;0.75; 0≦z≦0.3; 0.98≦m≦1.0; and 0&lt;n&lt;0.1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric ceramic composition, and particularly to a piezoelectric ceramic composition useful as a material for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators.

2. Background Art

Piezoelectric ceramic compositions predominantly comprising lead titanate zirconate (Pb(Ti_(x) Zr_(l-x))O₃) or lead titanate (PbTiO₃) have been widely used for piezoelectric ceramic elements such as piezoelectric ceramic filters. In the production step for these types of piezoelectric ceramic compositions, lead oxide is generally used. However, vaporization of lead oxide causes deteriorated uniformity in characteristics of the produced elements.

In contrast, piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate represented by a formula such as (K_(l-x-y) Na_(x) Li_(y))NbO₃ do not cause the above problem since they contain no lead oxide. Some such compositions comprising potassium sodium lithium niobate have a high electromechanical coupling coefficient K_(p) and are considered to be promising materials for producing piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators.

However, the piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate have a relative dielectric constant lower than that of lead titanate zirconate or lead titanate. Therefore, when they are used as materials for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators, impedance-matching with a circuit including the ceramic elements is poor and circuit design sometimes become difficult.

When a piezoelectric ceramic composition is used in a high-frequency region, the following problems arise. Since a conventional piezoelectric ceramic composition predominantly comprising lead titanate zirconate generally has a relatively high relative dielectric constant (approximately 1000-2000), impedance decreases in the high-frequency region of, for example, more than 100 MHz to cause difficulty in use thereof in such a high-frequency region.

In contrast, piezoelectric ceramic compositions predominantly comprising lead titanate (PbTiO₃) generally have a relative dielectric constant of approximately 200, which is lower than that of the above piezoelectric ceramic compositions predominantly comprising lead titanate zirconate. Therefore, the compositions comprising lead titanate (PbTiO₃) are known to be useful in the higher high-frequency region. However, an even lower relative dielectric constant is desired in consideration of use in a higher high-frequency region.

In addition, piezoelectric ceramic compositions predominantly comprising lead titanate zirconate or lead titanate have a resonance frequency of vibration in a thickness direction as low as approximately 2000-2500 Hz·m. Thus, when such a piezoelectric ceramic composition is processed into a thin slice so as to form a vibrator, the vibrator must be used in a limited frequency band.

In contrast, some of the piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate represented by a formula such as (K_(l-x-y) Na_(x) Li_(y))NbO₃ have a resonance frequency of vibration in a thickness direction as low as approximately 3000-3500 Hz·m and a relative dielectric constant of approximately 100, which is lower than that of lead titanate. Therefore, some of the compositions are known to be used as a material which has characteristics more advantageous than those of lead titanate zirconate or lead titanate in consideration of use thereof in the high-frequency region.

However, the piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate have a large temperature-dependent factor of resonance frequency of vibration in a thickness direction as high as approximately 150-300 ppm. This factor is referred to as fr-TC, and represents an important characteristic of a material for piezoelectric ceramic filters and piezoelectric ceramic oscillators. Therefore, the piezoelectric ceramic compositions have not yet been used widely in practice, as compared with lead titanate zirconate, lead titanate, etc.

The above-described temperature-dependent factor of resonance frequency of vibration in a thickness direction represented by fr-TC is calculated from the following equation:

    fr-TC=(fr.sub.max -fr.sub.min)/(fr.sub.20 ·100)

wherein fr_(max) represents the maximum resonance frequency of vibration in a thickness direction within a temperature range of -20° C. to +80° C.; fr_(min) represents a minimum resonance frequency of vibration in the thickness direction within a temperature range of -20° C. to +80° C.; and fr₂₀ represents a resonance frequency of vibration in the thickness direction at 20° C.

SUMMARY OF THE INVENTION

The present invention has been achieved in an attempt to solve the above-mentioned problems with piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate.

Accordingly, an object of the present invention is to provide a lead-free piezoelectric ceramic composition which has an enhanced relative dielectric constant of 1000 or more and an electromechanical coupling coefficient represented by K_(p) of a practically sufficient value, e.g., 25% or more.

Another object of the present invention is to provide a piezoelectric ceramic composition in which the temperature-dependent factor of resonance frequency represented by fr-TC is favorable, i.e., 100 ppm or less, and the relative dielectric constant is 180 or less, and which composition is suitably used in the high-frequency region and under conditions that the temperature-dependent factor of resonance frequency of vibration in a thickness direction is 3000 Hz·m or more.

Broadly, the present invention provides a piezoelectric ceramic composition represented by the formula:

    (1-n)(K.sub.l-x-y Na.sub.x Li.sub.y).sub.m (Nb.sub.l-z Ta.sub.z)O.sub.3 -nM1M2O.sub.3

wherein M1 represents a divalent metal element; M2 represents a tetravalent metal element; and x, y, z, m, and n satisfy the following conditions: 0.9≦x; y≦0.3; x+y≦0.75; 0≦z≦0.3; 0.98≦m≦1.0; and 0<n<0.1.

In a first aspect of the present invention, there is provided a piezoelectric ceramic composition which predominantly comprises a composition represented by the following formula:

    (l-n)(K.sub.l-x-y Na.sub.x Li.sub.y).sub.m (Nb.sub.l-z Ta.sub.z)O.sub.3 -nM1M2O.sub.3

wherein M1 represents a divalent metal element; M2 represents a tetravalent metal element; and x, y, z, m, and n satisfy the following conditions: 0.1≦x; y≦0.3; x+y<0.75; 0≦z≦0.3; 0.98≦m≦1.0; and 0<n<0.1.

Preferably, M1 is at least one element selected from the group consisting of Mg, Ca, Sr and Ba, and M2 is at least one element selected from the group consisting of Ti, Zr, Sn and Hf.

According to the first aspect of the present invention, there can be obtained a piezoelectric ceramic composition having excellent characteristics, e.g., a relative dielectric constant of 1000 or more, an electromechanical coupling coefficient represented by K_(p) of 25% or more and a Curie point of more than 200° C.

In a second aspect of the present invention, there is provided a piezoelectric ceramic composition which predominantly comprises a composition represented by the following formula:

    (l-n)(K.sub.l-x-y Na.sub.x Li.sub.y).sub.m (Nb.sub.l-z Ta.sub.z)O.sub.3 -nM1M2O.sub.3

wherein M1 represents a divalent metal element; M2 represents a tetravalent metal element; and x, y, z, m, and n satisfy the following conditions: x≦0.9; 0.02≦y≦0.3; 0.75≦x+y; 0≦z≦0.3; 0.98≦m≦1.0; and 0<n≦0.05.

Preferably, M1 is at least one element selected from the group consisting of Mg, Ca, Sr and Ba, and M2 is at least one element selected from the group consisting of Ti, Zr, Sn and Hf.

According to the second aspect of the present invention, there can be obtained a piezoelectric ceramic composition having excellent characteristics, e.g., a temperature-dependent factor of resonance frequency represented by fr-TC of 100 ppm or less and a Curie point of more than 200° C. and which composition is suitably used in the high-frequency region and under the conditions that the relative dielectric constant is 180 or less and the temperature-dependent factor of resonance frequency of vibration in a thickness direction is 3000 Hz·m or more.

By use of the piezoelectric ceramic composition of the first or the second aspect of the present invention, piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators are advantageously produced so as to meet the desired characteristics of the elements to be produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, the piezoelectric ceramic composition according to the present invention predominantly comprises a composition represented by the following formula.

    (l-n)(K.sub.l-x-y Na.sub.x Li.sub.y).sub.m (Nb.sub.l-z Ta.sub.z)O.sub.3 -nM1M2O.sub.3

With reference to the first aspect of the present invention, the reasons why the parameters are limited to the given ranges will next be described.

The x and y are limited such that 0.1<x and y≦0.3 because when x and y fall outside the ranges, sintered products are unsatisfactory. The x+y is limited such that x+y<0.75 because when x+y is 0.75 or more, the electromechanical coupling coefficient represented by K_(p) decreases to less than 25% to thereby result in difficulty in application of the composition to a material for piezoelectric ceramic filters, piezoelectric ceramic oscillators, etc.

The z is limited such that 0≦z≦0.3. When z falls outside the range, the Curie point decreases to 200° C. or less to cause a problem in variation of characteristics with respect to temperature change of elements formed of the piezoelectric ceramic composition according to the present invention.

The m is limited such that 0.98≦m≦1.0. When m falls outside the range, polarization treatment of a fired piezoelectric ceramic composition is difficult.

The n is limited such that 0<n<0.1. When n is 0.1 or more, the electromechanical coupling coefficient represented by K_(p) decreases to less than 25% to thereby result in difficulty in application of the composition to a material for piezoelectric ceramic filters, piezoelectric ceramic oscillators, etc.

With reference to the second aspect of the present invention, the reasons why the parameters are limited to the given ranges will next be described.

The x and y are limited such that x≦0.9 and 0.02≦y≦0.3 because when x and y fall outside the ranges, sintered products are unsatisfactory. The value x+y is limited such that 0.75≦x+y because when less than 0.75, the relative dielectric constant is in excess of 180, to thereby lose advantages in application of the composition in the high-frequency region.

The z is limited such that 0≦z≦0.3. When z falls outside the range, the Curie point decreases to 200° C. or less to cause a problem in variation of characteristics with respect to temperature change of elements formed of the piezoelectric ceramic composition according to the present invention.

The m is limited such that 0.98≦m≦1.0. When m falls outside the range, polarization treatment of a fired piezoelectric ceramic composition is difficult.

The n is limited such that 0<n≦0.05. When n is in excess of 0.05, the Curie point decreases to 200° C. or less to cause a problem in variation of characteristics with respect to temperature change of elements formed of the piezoelectric ceramic composition according to the present invention.

EXAMPLES

The present invention will next be described in detail by way of examples, which should not be construed as limiting the invention thereto.

Example 1

K₂ CO₃, Na₂ CO₃, Li₂ CO₃, Nb₂ O₅, Ta₂ O₅, CaCO₃, SrCO₃, BaCO₃, TiO₂, ZrO₂ and SnO₂ were provided as starting raw materials and were weighed so as to form compositions of the formula (l-n)(K_(l-x-y) Na_(x) Li_(y))_(m) (Nb_(l-z) Ta_(z))O₃ -nM1M2O₃ shown in Table 1. For each sample, the starting materials were wet-mixed in alcohol for about four hours by use of a ball mill, and the resultant mixture was dried and then calcined at 700-900° C. Subsequently, the dried mixture was roughly crushed, wet-milled with an appropriate amount of an organic binder for four hours by use of a ball mill, and passed through a 40-mesh sieve to thereby adjust the particle size of the milled powder.

Subsequently, the particle-size-adjusted powder was molded at 1000 kg/cm² into a disk having a diameter of 12 mm and a thickness of 1.2 mm, and the disk was fired at 1050° C.-1300° C. through a customary method for firing to thereby form a ceramic disk. Subsequently, silver electrodes were formed on both sides of the ceramic disk by applying and burning a silver paste through customary methods. The disk was subjected to polarization treatment by the application of a DC voltage of 2-10 kV/mm for 10-30 minutes in a 50° C.-150° C. insulating oil to thereby obtain a piezoelectric ceramic disk serving as a sample.

Relative dielectric constant, electromechanical coupling coefficient represented by K_(p) and Curie point were measured for the obtained samples. The results are shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________                                      Relative                                                                             Electromechanical                       Sample          x  y   z  m   n  Dielectric                                                                           coupling Curie Point                    No.  M1   M2    mol                                                                               mol mol                                                                               mol mol                                                                               Constant                                                                             coefficient K.sub.P                                                                     (° C.)                  __________________________________________________________________________     *1   Ca   Ti    0.7                                                                               0.1 0  1   0.05                                                                               850  22.0     430                            *2   --   --    0.6                                                                               0.1 0  1   0   580  36.5     400                             3   Ca   Ti    0.6                                                                               0.1 0  1   0.05                                                                              1010  31.0     370                             4   Ca   Ti    0.6                                                                               0.1 0  1   0.09                                                                              1280  29.5     310                            *5   Ca   Ti    0.6                                                                               0.1 0  1   0.1                                                                               1350  23.5     250                            *6   --   --    0.4                                                                               0.1 0  1   0   650  35.5     370                             7   Ca   Ti    0.4                                                                               0.1 0  1   0.05                                                                              1110  30.0     290                             8   Ca   Ti    0.4                                                                               0.1 0  1   0.09                                                                              1310  28.5     235                            *9   Ca   Ti    0.4                                                                               0.1 0  1   0.1                                                                               1420  24.5     215                            *10  --   --    0.1                                                                               0.1 0  1   0   350  28.0     405                            11   Ca   Ti    0.1                                                                               0.1 0  1   0.05                                                                              1005  27.0     365                            12   Ca   Ti    0.1                                                                               0.1 0  1   0.09                                                                              1220  25.5     300                            *13  Ca   Ti    0.1                                                                               0.1 0  1   0.1                                                                               1240  22.0     260                            *14  Ca   Ti    0.05                                                                              0.1 0  1   0.05                                                                              Poor sintering                                *15  Ca   Ti    0.4                                                                               0.4 0  1   0.05                                                                              Poor sintering                                *16  --   --    0.4                                                                               0.3 0  1   0   470  27.5     380                            17   Ca   Ti    0.4                                                                               0.3 0  1   0.05                                                                              1100  26.0     300                            18   Ca   Ti    0.4                                                                               0.3 0  1   0.09                                                                              1210  25.0     225                            *19  Ca   Ti    0.4                                                                               0.3 0  1   0.1                                                                               1270  21.5     205                            *20  --   --    0.4                                                                               0   0  1   0   420  37.0     375                            21   Ca   Ti    0.4                                                                               0   0  1   0.05                                                                              1010  32.0     295                            22   Ca   Ti    0.4                                                                               0   0  1   0.09                                                                              1240  29.5     265                            *23  Ca   Ti    0.4                                                                               0   0  1   0.1                                                                               1360  24.5     240                            *24  --   --    0.4                                                                               0   0.1                                                                               1   0   505  39.0     365                            25   Ca   Ti    0.4                                                                               0   0.1                                                                               1   0.05                                                                              1150  34.5     280                            26   Ca   Ti    0.4                                                                               0   0.1                                                                               1   0.09                                                                              1270  27.5     240                            *27  Ca   Ti    0.4                                                                               0   0.1                                                                               1   0.1                                                                               1370  24.5     230                            *28  --   --    0.4                                                                               0   0.3                                                                               1   0   570  36.0     315                            29   Ca   Ti    0.4                                                                               0   0.3                                                                               1   0.05                                                                              1190  30.5     235                            30   Ca   Ti    0.4                                                                               0   0.3                                                                               1   0.09                                                                              1320  26.0     215                            *31  Ca   Ti    0.4                                                                               0   0.3                                                                               1   0.1                                                                               1420  23.0     200                            *32  Ca   Ti    0.4                                                                               0   0.4                                                                               1   0.05                                                                              1500  34.0     160                            *33  --   --    0.4                                                                               0   0  0.98                                                                               0   425  40.5     380                            34   Ca   Ti    0.4                                                                               0   0  0.98                                                                               0.05                                                                              1020  38.0     290                            35   Ca   Ti    0.4                                                                               0   0  0.98                                                                               0.09                                                                              1210  30.5     265                            *36  Ca   Ti    0.4                                                                               0   0  0.98                                                                               0.1                                                                               1280  24.5     240                            *37  Ca   Ti    0.4                                                                               0   0  0.97                                                                               0.05                                                                              Not polarized                                 38   Ba   Ti    0.4                                                                               0   0  1   0.05                                                                              1100  31.5     280                            39   Ba   Ti    0.4                                                                               0   0  1   0.09                                                                              1240  27.5     250                            *40  Ba   Ti    0.4                                                                               0   0  1   0.1                                                                               1400  23.0     235                            41   Ca.sub.1/2 Sr.sub.1/2                                                               Ti    0.4                                                                               0   0  1   0.05                                                                              1005  32.0     320                            42   Ca.sub.1/2 Sr.sub.1/2                                                               Ti    0.4                                                                               0   0  1   0.09                                                                              1070  28.0     280                            *43  Ca.sub.1/2 Sr.sub.1/2                                                               Ti    0.4                                                                               0   0  1   0.1                                                                               1105  24.0     255                            44   Ca   Zr    0.4                                                                               0   0  1   0.05                                                                              1045  33.5     305                            45   Ca   Zr    0.4                                                                               0   0  1   0.09                                                                              1125  28.0     265                            *46  Ca   Zr    0.4                                                                               0   0  1   0.1                                                                               1220  24.5     250                            47   Ca   Ti.sub.1/2 Sn.sub.1/2                                                                0.4                                                                               0   0  1   0.05                                                                              1200  30.0     280                            48   Ca   Ti.sub.1/2 Sn.sub.1/2                                                                0.4                                                                               0   0  1   0.09                                                                              1410  26.0     235                            *49  Ca   Ti.sub.1/2 Sn.sub.1/2                                                                0.4                                                                               0   0  1   0.1                                                                               1500  22.5     200                            __________________________________________________________________________

In Table 1, samples marked with * fall outside the scope of the present invention.

Samples in Table 1 satisfying all the following conditions: 0.1≦x; y≦0.3; x+y<0.75; 0≦z≦0.3; 0.98≦m≦1.0; and 0<n<0.1 exhibit excellent characteristics. Briefly, all samples according to the present invention (those not marked with *) have a relative dielectric constant of 1000 or more, an electromechanical coupling coefficient represented by K_(p) of 25% or more and a Curie point of more than 200° C.

In contrast, Sample Nos. 14 and 15, which fail to satisfy either the condition 0.1≦x or the condition y≦0.3, exhibited poor sinterability.

Sample No. 1, which satisfies the condition 0.1≦x or and? y≦0.3 and does not satisfy the condition x+y<0.75, has an electromechanical coupling coefficient represented by K_(p) of 22.0%. Sample No. 1 also has a relative dielectric constant of 850. As shown by the data, Sample No. 1 does not attain the relative dielectric constant of 1000 or more and electromechanical coupling coefficient represented by K_(p) of 25% or more.

Sample No. 32, which does not satisfy the condition 0≦z≦0.3, has a Curie point of 160° C. and thus does not attain a Curie point of more than 200° C.

Sample No. 37, which does not satisfy the condition 0.98≦m≦1.0, does not attain desirable polarization.

Among the samples which do not satisfy the condition 0<n<0.1, Sample Nos. 5, 9, 13, 19, 23, 27, 31, 36, 40, 43, 46 and 49 having an n of 0.1 or more have an electromechanical coupling coefficient represented by K_(p) of lower than 25% and Sample Nos. 2, 6, 10, 16, 20, 24, 28 and 33 having an n of 0 do not attain a relative dielectric constant of 1000 or more.

In Example 1, at least one species selected from the group consisting of Ca, Ba and Sr was used as M1; and at least one species selected from the group consisting of Ti, Zr and Sn was used as M2. However, the inventors confirmed that equivalent effects can be realized when Mg is used as M1 and Hf is used as M2. Other divalent metals and tetravalent metals may also be used as M1 and M2, respectively.

As demonstrated by this Example, the first aspect of the present invention provides a piezoelectric ceramic composition exhibiting excellent characteristics; i.e., an enhanced relative dielectric constant of 1000 or more, an electromechanical coupling coefficient represented by K_(p) of 25% or more, and a Curie point of higher than 200° C. Therefore, by use of the piezoelectric ceramic composition according to the first aspect of the present invention, piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators are advantageously produced.

Example 2

K₂ CO₃, Na₂ CO₃, Li₂ CO₃, Nb₂ O₅, Ta₂ O₅, CaCO₃, SrCO₃, BaCO₃, TiO₂, ZrO₂ and SnO₂ were provided as starting raw materials and were weighed so as to form compositions of the formula (ln)(K_(l-x-y) Na_(x) Li_(y))_(m) (Nb_(l-z) Ta_(z))O₃ -nM1M2O₃ shown in Tables 2 and 3. For each sample, the starting materials were wet-mixed in alcohol for about four hours by use of a ball mill, and the resultant mixture was dried and then calcined at 700-900° C. Subsequently, the dried mixture was roughly crushed, wet-milled with an appropriate amount of an organic binder for four hours by use of a ball mill and passed through a 40-mesh sieve to thereby adjust the particle size of the milled powder.

Subsequently, the particle-size-adjusted powder was molded at 1000 kg/cm² into a disk having a diameter of 12 mm and a thickness of 1.2 mm, and the disk was fired at 1050° C.-1300° C. through a customary method for firing to thereby form a ceramic disk. Subsequently, silver electrodes were formed on both sides of the ceramic disk by applying and burning a silver paste through customary methods. The disk was subjected to polarization treatment by the application of a DC voltage of 2-10 kV/mm for 10-30 minutes in a 50° C.-150° C. insulating oil, to thereby obtain a piezoelectric ceramic disk serving as a sample.

                  TABLE 2                                                          ______________________________________                                         Sample               x     y     z     m     n                                 No.    M1     M2     mol   mol   mol   mol   mol                               ______________________________________                                         *101   Ca     Ti     0.95  0.05  0     1     0.01                              *102   --     --     0.9   0.1   0     1     0                                 103    Ca     Ti     0.9   0.1   0     1     0.01                              104    Ca     Ti     0.9   0.1   0     1     0.05                              *105   Ca     Ti     0.9   0.1   0     1     0.06                              *106   --     --     0.85  0.15  0     1     0                                 107    Ca     Ti     0.85  0.15  0     1     0.01                              108    Ca     Ti     0.85  0.15  0     1     0.02                              109    Ca     Ti     0.85  0.15  0     1     0.05                              *110   Ca     Ti     0.85  0.15  0     1     0.06                              *111   --     --     0.8   0.2   0     1     0                                 112    Ca     Ti     0.8   0.2   0     1     0.01                              113    Ca     Ti     0.8   0.2   0     1     0.05                              *114   Ca     Ti     0.8   0.2   0     1     0.06                              *115   --     --     0.7   0.3   0     1     0                                 116    Ca     Ti     0.7   0.3   0     1     0.01                              117    Ca     Ti     0.7   0.3   0     1     0.05                              *118   Ca     Ti     0.7   0.3   0     1     0.06                              *119   Ca     Ti     0.9   0     0     1     0.01                              *120   --     --     0.88  0.02  0     1     0                                 121    Ca     Ti     0.88  0.02  0     1     0.01                              122    Ca     Ti     0.88  0.02  0     1     0.05                              *123   Ca     Ti     0.88  0.02  0     1     0.06                              *124   --     --     0.75  0.15  0     1     0                                 125    Ca     Ti     0.75  0.15  0     1     0.01                              126    Ca     Ti     0.75  0.15  0     1     0.05                              *127   Ca     Ti     0.75  0.15  0     1     0.06                              *128   --     --     0.6   0.3   0     1     0                                 129    Ca     Ti     0.6   0.3   0     1     0.01                              130    Ca     Ti     0.6   0.3   0     1     0.05                              *131   Ca     Ti     0.6   0.3   0     1     0.06                              *132   Ca     Ti     0.5   0.4   0     1     0.01                              *133   --     --     0.73  0.02  0     1     0                                 134    Ca     Ti     0.73  0.02  0     1     0.01                              135    Ca     Ti     0.73  0.02  0     1     0.05                              *136   Ca     Ti     0.73  0.02  0     1     0.06                              *137   --     --     0.6   0.15  0     1     0                                 ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Sample                  x    y     z    m     n                                No.   M1       M2       mol  mol   mol  mol   mol                              ______________________________________                                         138   Ca       Ti       0.6  0.15  0    1     0.01                             139   Ca       Ti       0.6  0.15  0    1     0.05                             *140  Ca       Ti       0.6  0.15  0    1     0.06                             *141  --       --       0.45 0.3   0    1     0                                142   Ca       Ti       0.45 0.3   0    1     0.01                             143   Ca       Ti       0.45 0.3   0    1     0.05                             *144  Ca       Ti       0.45 0.3   0    1     0.06                             *145  Ca       Ti       0.4  0.3   0    1     0.01                             *146  --       --       0.88 0.02  0.1  1     0                                147   Ca       Ti       0.88 0.02  0.1  1     0.01                             148   Ca       Ti       0.88 0.02  0.1  1     0.05                             *149  Ca       Ti       0.88 0.02  0.1  1     0.06                             *150  --       --       0.88 0.02  0.3  1     0                                151   Ca       Ti       0.88 0.02  0.3  1     0.01                             152   Ca       Ti       0.88 0.02  0.3  1     0.05                             *153  Ca       Ti       0.88 0.02  0.3  1     0.06                             *154  Ca       Ti       0.88 0.02  0.4  1     0.01                             *155  --       --       0.88 0.02  0    0.99  0                                156   Ca       Ti       0.88 0.02  0    0.99  0.01                             157   Ca       Ti       0.88 0.02  0    0.99  0.05                             *158  Ca       Ti       0.88 0.02  0    0.99  0.06                             *159  --       --       0.88 0.02  0    0.98  0                                160   Ca       Ti       0.88 0.02  0    0.98  0.01                             161   Ca       Ti       0.88 0.02  0    0.98  0.05                             *162  Ca       Ti       0.88 0.02  0    0.98  0.06                             *163  Ca       Ti       0.88 0.02  0    0.97  0.01                             164   Ba       Ti       0.88 0.02  0    1     0.01                             165   Ba       Ti       0.88 0.02  0    1     0.05                             *166  Ba       Ti       0.88 0.02  0    1     0.06                             167   Ca.sub.1/2 Sr.sub.1/2                                                                   Ti       0.88 0.02  0    1     0.01                             168   Ca.sub.1/2 Sr.sub.1/2                                                                   Ti       0.88 0.02  0    1     0.05                             *169  Ca.sub.1/2 Sr.sub.1/2                                                                   Ti       0.88 0.02  0    1     0.06                             170   Ca       Zr       0.88 0.02  0    1     0.01                             171   Ca       Zr       0.88 0.02  0    1     0.05                             *172  Ca       Zr       0.88 0.02  0    1     0.06                             173   Ca       Ti.sub.1/2 Sn.sub.1/2                                                                   0.88 0.02  0    1     0.01                             174   Ca       Ti.sub.1/2 Sn.sub.1/2                                                                   0.88 0.02  0    1     0.05                             *175  Ca       Ti.sub.1/2 Sn.sub.1/2                                                                   0.88 0.02  0    1     0.06                             ______________________________________                                    

Relative dielectric constant, electromechanical coupling coefficient in vibration in a thickness direction represented by K_(t), resonance frequency constant of vibration in a thickness direction represented by N, temperature-dependent factor of resonance frequency of vibration in a thickness direction represented by fr-TC and Curie point were measured for the obtained samples shown in Table 2 and 3. The results are shown in Tables 4 and 5.

                  TABLE 4                                                          ______________________________________                                                                          Temperature-                                                 Electro-  Resonance                                                                              Dependent                                                    mechanical                                                                               Frequency                                                                              Factor of                                           Relative Coupling  Constant of                                                                            Resonance                                                                               Curie                                Sample                                                                               Dielectric                                                                              Coefficient                                                                              Vibration                                                                              Frequency                                                                               Point                                No.   Constant (%)       (Hz · m)                                                                      (ppm)    (° C.)                        ______________________________________                                         *101  Poor sintering                                                           *102  105      42        3460    170      420                                  103   115      40        3570    95       380                                  104   145      38        3640    85       220                                  *105  160      24        3820    85       180                                  *106   90      48        3620    175      425                                  107    95      43        3710    90       410                                  108   100      42        3740    90       400                                  109   120      40        3770    90       270                                  *110  140      29        3850    90       190                                  *111   90      45        3640    200      435                                  112    95      41        3710    95       420                                  113   115      35        3700    90       285                                  *114  125      22        3740    85       195                                  *115   85      35        3580    250      440                                  116    95      32        3600    100      415                                  117   110      25        3600    95       290                                  *118  115      18        3610    100      195                                  *119  Poor sintering                                                           *120  110      48        3420    220      400                                  121   125      44        3425    90       380                                  122   145      37        3420    85       265                                  *123  160      30        3440    85       190                                  *124  105      40        3405    260      410                                  125   105      32        3390    95       395                                  126   125      29        3390    85       285                                  *127  125      26        3405    90       190                                  *128  105      38        3400    255      410                                  129   110      33        3400    100      390                                  130   120      26        3385    95       290                                  *131  120      20        3370    95       185                                  *132  Poor sintering                                                           *133  165      38        3200    310      400                                  134   165      29        3220    100      350                                  135   175      26        3240    95       225                                  *136  175      18        3230    100      155                                  *137  150      40        3235    260      415                                  ______________________________________                                    

                  TABLE 5                                                          ______________________________________                                                                          Temperature-                                                 Electro-  Resonance                                                                              Dependent                                                    mechanical                                                                               Frequency                                                                              Factor of                                           Relative Coupling  Constant of                                                                            Resonance                                                                               Curie                                Sample                                                                               Dielectric                                                                              Coefficient                                                                              Vibration                                                                              Frequency                                                                               Point                                No.   Constant (%)       (Hz · m)                                                                      (ppm)    (° C.)                        ______________________________________                                         138   155      29        3240    90       385                                  139   170      24        3260    85       280                                  *140  170      20        3260    85       185                                  *141  110      37        3300    195      425                                  142   120      31        3305    100      395                                  143   125      26        3415    100      305                                  *144  125      20        3385    100      195                                  *145  200      35        3085    100      400                                  *146  130      48        3220    260      350                                  147   135      40        3205    100      325                                  148   145      35        3215    95       205                                  *149  170      32        3260    100      155                                  *150  160      47        3205    245      270                                  151   170      39        3185    95       245                                  152   175      32        3170    95       205                                  *153  175      29        3170    95       160                                  *154  180      35        3015    100      165                                  *155  110      49        3420    220      395                                  156   125      46        3425    90       380                                  157   145      40        3415    85       265                                  *158  160      36        3445    85       185                                  *159  110      47        3400    220      390                                  160   125      43        3415    90       375                                  161   140      37        3405    85       255                                  *162  155      29        3430    85       180                                  *163  Not polarized                                                            164   125      40        3325    95       380                                  165   140      34        3320    95       245                                  *166  150      28        3315    90       180                                  167   120      40        3475    95       380                                  168   135      32        3430    85       255                                  *169  135      27        3420    85       185                                  170   125      43        3440    90       380                                  171   145      37        3455    85       265                                  *172  155      27        3470    85       180                                  173   135      47        3355    95       355                                  174   165      40        3320    95       225                                  *175  170      32        3305    95       165                                  ______________________________________                                    

In Tables 2 to 5, samples marked with * fall outside the scope of the present invention.

Those samples in Tables 2 and 3 satisfying all the following conditions: x≦0.9; 0.02≦y≦0.3; 0.75≦x+y; 0≦z≦0.3; 0.98≦m≦1.0; and 0<n≦0.05 exhibit excellent characteristics. Briefly, as shown in Tables 4 and 5, all samples according to the present invention (those not marked with *) have a relative dielectric constant of 180 or less, a resonance frequency constant of vibration in a thickness direction of 3000 Hz·m or more, a temperature-dependent factor of resonance frequency of 100 ppm or less and a Curie point of more than 200° C.

In contrast, Sample Nos. 101, 119 and 132, which fail to satisfy either the condition x≦0.9 or the condition 0.02≦y≦0.3 exhibited poor sinterability.

Sample No. 145, which does not satisfy the condition 0.75≦x+y, has a relative dielectric constant of 200 and thus does not attain a relative dielectric constant of 180 or less.

Sample No. 154, which does not satisfy the condition 0≦z≦0.3, has a Curie point of 165° C. and thus does not attain a Curie point of more than 200° C.

Sample No. 163, which does not satisfy the condition 0.98≦m≦1.0, does not attain desirable polarization.

Among the samples which do not satisfy the condition 0<n≦0.05, Sample Nos. 105, 110, 114, 118, 123, 127, 131, 136, 140, 144, 149, 153, 158, 162, 166, 169, 172 and 175 having an n of greater than 0.05 have a Curie point of 200° C. or less, and Sample Nos. 102, 106, 111, 115, 120, 124, 128, 133, 137, 141, 146, 150, 155 and 159 having an n of 0 have a temperature-dependent factor of resonance frequency in great excess of 100 ppm.

In Example 2, at least one species selected from the group consisting of Ca, Ba and Sr was used as M1; and at least one species selected from the group consisting of Ti, Zr and Sn was used as M2. However, the inventors confirmed that equivalent effects can be realized when Mg and/or Pb are used as M1 and Hf is used as M2. Other divalent metals and tetravalent metals may also be used as M1 and M2, respectively.

As demonstrated by this Example, the second aspect of the present invention provides a piezoelectric ceramic composition exhibiting excellent characteristics; i.e., a relative dielectric constant of 180 or less and a temperature-dependent factor of resonance frequency of vibration in a thickness direction of 3000 Hz·m or more, proving that the composition can be advantageously used in a high frequency region. Moreover, the composition has a temperature-dependent factor of resonance frequency represented by fr-TC of 100 ppm or less and a Curie point of higher than 200° C. Therefore, by use of the piezoelectric ceramic composition according to the second aspect of the present invention, piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators are advantageously produced.

Although the present invention has been described above with reference to nonlimiting examples, those skilled in the art will recognize that many variations on the piezoelectric ceramic composition of the present invention are possible. Such variations are intended to be within the scope of the present invention. 

What is claimed is:
 1. A piezoelectric ceramic composition represented by the formula:

    (1-n)(K.sub.1-x-y Na.sub.x Li.sub.y).sub.m (Nb.sub.1-z Ta.sub.z)O.sub.3 -nM1M2O.sub.3

wherein M1 represents a divalent metal element; M2 represents a tetravalent metal element; and x, y, z, m, and n satisfy the following conditions: 0.9≦x; y≦0.3; x+y≦0.75; 0≦z≦0.3; 0.98≦m<1.0; and 0<n<0.1.
 2. A piezoelectric ceramic composition according to claim 1, wherein 0.1≦x, and x+y<0.75.
 3. A piezoelectric ceramic composition according to claim 2, wherein M1 is at least one element selected from the group consisting of Mg, Ca, Sr and Ba, and M2 is at least one element selected from the group consisting of Ti, Zr, Sn and Hf.
 4. A piezoelectric ceramic composition according to claim 3, wherein M1 is at least one element selected from the group consisting of Ca, Sr and Ba, and M2 is at least one element selected from the group consisting of Ti, Zr and Sn.
 5. A piezoelectric ceramic composition according to claim 4, wherein M1 is Ca and M2 is Ti.
 6. A piezoelectric ceramic composition according to claim 1, wherein 0.9≦x and 0<n<0.05 and 0.02≦y≦0.3.
 7. A piezoelectric ceramic composition according to claim 6, wherein M1 is at least one element selected from the group consisting of Mg, Ca, Sr and Ba, and M2 is at least one element selected from the group consisting of Ti, Zr, Sn and Hf.
 8. A piezoelectric ceramic composition according to claim 7, wherein M1 is at least one element selected from the group consisting of Ca, Sr and Ba, and M2 is at least one element selected from the group consisting of Ti, Zr and Sn.
 9. A piezoelectric ceramic composition according to claim 8, wherein M1 is Ca and M2 is Ti. 